In laboratory analyses, it is often necessary to put a predetermined quantity of a liquid substance such as a serum or a reagent into a receptacle. This is commonly achieved by means of a graduated pipette which is used to suck up a certain quantity of liquid; thereafter, the predetermined quantity of liquid is dispensed by pressure.
However, when the dose to be dispensed is very small, this method suffers from being inaccurate.
Such a pipette enables quantities of liquid of about 25 microliters to be drawn off to within 5%. Such a quantity constitutes a considerable degree of inaccuracy, in particular in the event that the liquid to be drawn off is a reagent intended make another substance react for the purposes of analysis, e.g. a serum. In view of the high cost of certain reagents, research is being directed into decreasing the quantities of reagent used to as little as possible so as to make savings.
U.S. Pat. No. 4,874,114 describes a pipette for dispensing a predetermined dose of a liquid, which pipette is constituted by a hollow hemisphere fixed at one end of a rod. The hemisphere defines a volume corresponding to a certain quantity of liquid to be drawn off.
The volume is filled with a liquid by rotating the pipette at high speed about an axis by means of the rod. The speed of rotation is such that the centrifugal forces obtained are sufficient, for a liquid of given viscosity, to fill the above-defined volume entirely. By holding the hemisphere in a certain position relative to the vertical, the hemisphere can be filled with liquid when the rotary motion is applied. This liquid is ejected from the sphere when the hemisphere takes up another position relative to the vertical, and when the same rotary motion is applied.
That device can dispense a dose of liquid of about 5 microliters to within 0.3%.
The main drawback with that device is that it is quite bulky because the hemisphere must be able to tilt about an axis through an angle of about 3 degrees on either side of the vertical. Therefore, it is not possible to draw off/dispense a predetermined quantity of liquid from/into a receptacle having an access which has a small amount of clearance, e.g. a test tube.
In particular, an object of the present invention is to mitigate this drawback.
To be more precise, one of the objects of the invention is to provide apparatus for drawing off and dispensing a predetermined quantity of a liquid, which apparatus can be used in receptacles having small access openings.